first you should check /var/log/zimbra.log or log from postfix somewhere in /opt/zimbra/log for any output relevant to this error. You could connect to Zimbra's SMTP with telnet as seen in the "Test" paragraph in the wiki article and watch what goes into the logs about your try.
If you can't make out anything from that, you could have made a typo somewhere in the files you created or modified, please check that.
And finally, if you don't find anything wrong, please post some relevant output from the logs and the lines from main.cf you modified so we could try to pinpoint the problem.